The manufacture of filaments suited for optical data transmission comprises drawing of a so called preform which is formed of a glass material, usually of a glass rod made of an alloyed quartz and collapsed by heating after a gas treatment, and drawn particularly in a drawing furnace into a thin filament. In order to protect the surface of the filament from getting scratched during subsequent handling before the filament is coated with a secondary coating, it is known to apply a primary coating to the filament immediately after it is drawn.
The primary coating affects the properties of the filament to a great extent as even minor scratches and other such figures impair the strength properties of the filament. Therefore the primary coating has to surround the filament at all points if the aimed final result is to be obtained. The centricity of the primary coating is also of great importance because an acentric structure, i.e. a structure in which the thickness of the coating varies in the peripheral and longitudinal direction of the filament, results in poor thermal properties, for instance.
A great number of solutions have been developed for the formation of the primary coating. One typical solution is the use of an open crucible. In this known procedure the filament is drawn through a crucible filled with a coating material, with the result that the coating material adheres to the surface of the filament. An example of such solutions would be the procedure disclosed in U.S. Pat. No. 4,349,587. However, a method utilizing an open crucible is restricted in that the drawing speed of the filament cannot be made sufficiently high.
In an attempt the achieve higher drawing speeds, pressurized crucibles have later been taken into use. By means of such crucibles, it has been possible to considerably increase the drawing speed as compared to open systems. With an increased drawing speed, the drawbacks of an acentric coating have caused problems. To achieve improvement in this matter, various solutions have been brought out to extremely, accurately guide the filament and to cause the coating material to be distributed on the surface of the filament as evenly as possible.
Another example of prior art solutions would be the procedure disclosed in U.S. Pat. No. 4,409,263, wherein coating material is introduced into a crucible under a pressure such that an upward flow of the coating material is formed, and no disadvantageous air bubbles are carried with the filament moving at a high speed. The solution according to U.S. Pat. No. 4,409,263, however, has a disadvantage that it is complicated in structure and operation, whereby the reliability of its operation is not the best possible. Said patent takes no account of the symmetric distribution of the coating material for improving the centricity of the coating.
U.S. Pat. No. 4,439,467 teaches a symmetric material distribution in which the material is introduced into the chamber through a plurality of holes. In this case, too, the material is introduced into the orifice through a fixed number of holes so that an optimal evenness is not obtained. Furthermore, the connection is carried out by means of several bolts wherefore it is difficult to ensure that the temperature of the material is even. It is to be noted that the viscosity varies with the temperature and so possibly does the centricity, too. One further disadvantage is the variation of the pressure (impurities in the channels, manufacturing tolerances, differences in pipe lengths), which affects adversely the symmetrical material distribution, thus resulting in an inferior centricity.